Considerations in the Protection of Cellular Radio Sites
The need for effective lightning and surge protection on telecommunications towers regardless of configuration and geographic location can not be underestimated. This paper presents evidence to support the above statement.
1. Introduction
The merits of installing effective lightning and surge protection on telecommunications towers, along with adopting good grounding and bonding practices are well established.
Tower structures are often elevated above their surroundings in order to maximize coverage, thereby making them attractive lightning receptors to the vagaries of the Cloudto- Ground lightning discharge. The conventional Lightning Protection System (LPS) consisting of strike receptor, lightning downconductor and low impedance ground termination system is well defined in standards such as NFPA 7801 and IEC 62305.
Standards such as IEC 61643 on the other hand, cover the need for overvoltage surge protective devices being installed to safeguard sensitive electronic equipment. With the advent of relocating the radio units from the base shelter to the towertop, the exposure to the influences of the electro-magnetic impulse created by direct or nearby lightning discharges has also increased significantly. Such tower-top systems are particularly vulnerable to induced voltages onto DC power feeders.
This paper sets out to explore a number of topical questions commonly posed regarding the scope of protection needed for different sites and locations. It seeks to provide background to each question, explaining the mechanism of the risk involved, and the method best suited to reduce this. It also covers aspects such as whether protection is required in geographic locations where isokeraunic levels are low, whether protection is needed when the site is surrounded by structures of equal or greater elevation, whether it is necessary to protect both the base station and the remote radio head, and how best to protect against the direct and indirect strike.
Interesting material dealing with some non–typical storm data is also presented to provide a degree of caution to the design engineer that even in locations where the annual average of thunder day activity is generally low – such as the west coast of the USA – occasionally atypical events of nature can occur and cause just as much damage in a few hours as would normally only be expected in locations of high annual isokeraunic activity.
Finally, the paper introduces the concept of “zones of protection” as defined in the IEC 62305 series of lightning standards, and describes how this practical approach (using magnetic screening and surge protection measures) can be applied to establish “safe” zones for the RRH and BBU, where exposure is reduced to within their operation withstand level.
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